An electrophotographic image forming apparatus forms an image by developing an electrostatic latent image formed on a photosensitive member with a toner. The image forming apparatus includes an optical scanning device. An electrostatic latent image is formed on the photosensitive member by scanning with laser light emitted from the optical scanning device on the basis of image data. The optical scanning device includes a rotatable polygonal mirror that deflects laser light emitted from a light source, and an optical member such as a lens or a mirror that guides the laser light deflected by the rotatable polygonal mirror onto the photosensitive member.
The photosensitive characteristics of the surface of the photosensitive member slightly vary depending on the position of the surface of the photosensitive member. Even if the photosensitive member is exposed to laser light with the same light quantity, the density of an output image may be uneven due to the unevenness of the photosensitive characteristics of the surface of the photosensitive member.
To address the problem, PTL 1 discloses an image forming apparatus that corrects image data in accordance with a scanning position (exposure position) with laser light on a photosensitive member. With the image forming apparatus described in PTL 1, the unevenness of the density of the output image due to the unevenness of the photosensitive characteristics of the surface of the photosensitive member can be restricted.